The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Cold heavy oil production with sand (CHOPS) is one of the many methods currently employed to extract viscous heavy oil from deposits in Canada. Not all fields or local reservoirs are amenable to this technique, but due to its low cost it is often the method of choice whenever it can be applied.
CHOPS is a well documented technique and is a standard method of producing heavy oil in Northern Alberta and Saskatchewan. Further details of this technique can be found in http://www.energy.gov.ab.ca/docs/oilsands/pdfs/RPT_Chops_chptr3.pdf. It includes the deliberate initiation of sand influx during the completion procedure, maintenance of sand influx during the productive life of the well, separation of the sand from the oil, and finally the disposal of the sand. No sand exclusion devices (screens, liners, gravel packs, etc.) are used in the wellbores, and no filters, cyclones or high pressure separators are used at the surface. The sand is produced along with oil water and gas, and separated from the oil by settling before being cleaned and sent to a facility for upgrading to a synthetic crude.
One stimulation treatment routinely performed on oil and gas wells in low-permeability reservoirs is hydraulic fracturing. Specially engineered fluids are pumped at high pressure and rate into the reservoir interval to be treated, causing a vertical fracture to open. The wings of the fracture extend away from the wellbore in opposing directions according to the natural stresses within the formation. Proppant, such as grains of sand of a particular size, can be mixed with the treatment fluid keep the fracture open when the treatment is complete. Hydraulic fracturing creates high-conductivity communication with a large area of formation and bypasses any damage that may exist in the near-wellbore area.
Fracture acidizing (sometimes called ‘acid frac’) is a variation on the hydraulic fracturing well-stimulation operation in which acid, usually hydrochloric [HCl], is injected into a carbonate formation at a pressure above the formation-fracturing pressure. Flowing acid tends to etch the fracture faces in a non-uniform pattern, forming conductive channels that remain open without a propping agent after the fracture closes. The length of the etched fracture limits the effectiveness of an acid-fracture treatment. The fracture length depends on acid leakoff and acid spending. If acid fluid-loss characteristics are poor, excessive leakoff will terminate fracture extension. Similarly, if the acid spends too rapidly, the etched portion of the fracture will be too short. The major problem in fracture acidizing is the development of wormholes in the fracture face; these wormholes increase the reactive surface area and cause excessive leakoff and rapid spending of the acid. To some extent, this problem can be overcome by using inert fluid-loss additives to bridge wormholes or by using viscosified acids. Fracture acidizing is also called acid fracturing or acid-fracture treatment.
Reactive chemical systems have been considered for stimulating the diatomite formations in California (note that these are not produced by CHOPS).
It is an object of the invention to provide a technique for improving oil recovery that can be used in heavy oil formations without some or all of the problems of the previous techniques.